A power supply cap visibly needs replacing. Before I start ordering parts, I hope to learn more about why this unit is not working. I would like to at least check where there should be voltage. There is voltage at F-400 and F-401.

Line voltage is 121.7 VAC

Voltages from the power supply caps are +25.75 and -25.5. They should be +/-22. I suppose the higher line voltage probably causes that.

Here are pics from the schematic. I find various voltages (12-15-18 range) at some locations and not others. Where can I look for voltage that would help determine why this thing is dead?

As you can see, I am just learning about electronics. And before you ask, I know about capacitors and how to drain them. I am VERY careful. I have a good multimeter and various leads. I have used soldering pens. I am ignorant, but not stupid.

Yes, I have the larger schematic, but it's really fuzzy which is why I took those six pictures.

To answer your question, I can see the power section is sending power to the fuses. There is no output from the preamp. None whatsoever. It is completely dead. I want to know why. What would you check next? Here is the power supply which, as I mentioned, seems to be working.

Have you cycled & deoxidized the loudness and tape monitor switches? The stereo/mono switch? What input are you applying signal to?

It's a pretty simple preamp, pair of transistors in a differential pair at the input that's fed by the bass/treble circuit and then a single transistor for gain. There are test points for the +/- 15V supply rails that power those transistors.

Okay this is very frustrating. I have these six images in a folder on my computer. They all are right side up. When I post them, five are upside down. When I rotate those five, on my computer, save them as upside down files and put them in the opening post of this thread. They are still upside down. I post them right side up, and something inverts them. I post them already inverted, and they get posted inverted.

Don't worry about the inverted images. Good that your have +/-25VDC from the caps. They may not be filtering well (letting some AC through), given the visual degradation. You can do a quick check of that by measuring across them (just like you did for the +/-25VDC), but with your meter on AC. Good values would be no more than several 10's of mVAC. If you have 100s of mVAC or more, they will cause hum and need to be replaced. But even that would probably not make the unit not have output.

Alan can provide better advice in sorting through the circuit than I can, but a problem commonly reported on these (although mine has been ok) is the RA400 regulator, which provides the + part of the 15V circuit. You can find the RA400 in the lower right of the main Amp schematic. There, you can see that the +15VDC appears on the + side of C400, where you can easily measure it. Similarly, the -15VDC appears on the - side of C401, where it can be checked (always measure against a ground point, e.g., somewhere on the chassis). I would check those two first, to see if the +/-15V circuit is out.

If you need to know the locations, on the Amp board, RA400 is located on the edge of the board in the section marked "left amp", not far from fuse F400, which you already know. C400 and C401 are located in the middle of the board, nearly centered between the silkscreen markings for "left amp" and "right amp".

Another thing to be aware of is that there are "early" and "late" revisions, so check your serial number and make sure you are using the right diagrams. "Late" models are ones with serial numbers JO 07000 and higher or YO 01000 and higher.

Have you cycled & deoxidized the loudness and tape monitor switches? The stereo/mono switch? What input are you applying signal to?

It's a pretty simple preamp, pair of transistors in a differential pair at the input that's fed by the bass/treble circuit and then a single transistor for gain. There are test points for the +/- 15V supply rails that power those transistors.

John

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Thanks, John. Just now, running this through the Mitsubishi (70 WP) with the Mitsubishi cranked, I put a music source into all three inputs (Aux, Phono, Tape In) and each time I cranked the Avent's volume and worked all the switches back and forth. No sound whatsoever. No clicking, no hum, no nothing. It is as if this unit is not connected to the Mitsubishi. The Preamp Out on this Advent is entirely DEAD.

You're awesome, northpaw. Awesome. Thank you. Between the "Left Amp" and "Right Amp" printing there are two pairs of cylindrical caps. Two tiny ones and two small, but not tiny, ones. I think C400 and C401 are 100uf caps, the larger of the two pairs. I cannot see which is which, as the caps cover the silkscreen. In any case, the voltages at either side of those two caps are:

You're awesome, northpaw. Awesome. Thank you. Between the "Left Amp" and "Right Amp" printing there are two pairs of cylindrical caps. Two tiny ones and two small, but not tiny, ones. I think C400 and C401 are 100uf caps, the larger of the two pairs. I cannot see which is which, as the caps cover the silkscreen. In any case, the voltages at either side of those two caps are:

This is my redraw of the voltage regulator circuit for easier understanding:

The +ve 15V is from a 78M15 voltage regulator. Q401, 402 and 403 form an inverter amp to create -15V. The voltage is set by R401 and 402. Q402 has the gate and source connected together so it becomes a constant current source ( actually sink) of about 6mA ( look at datasheet, current sink when Vgs=0 is about 6mA).

You don't have -15V, things you have to check is:

1) Emitter of Q403 at "B" should be about +0.7V. measure and tell me the reading.
2) The base of Q401 at "A" should be about -15.7V, measure and tell me the reading.

My first suspicion is something wrong with Q402. Look at this area on the pcb for cold solder, touch up with soldering iron to make sure all the soldering is good. If it's still have problem, something is wrong. Report back and we do more.

Between the "Left Amp" and "Right Amp" printing there are two pairs of cylindrical caps. Two tiny ones and two small, but not tiny, ones. I think C400 and C401 are 100uf caps, the larger of the two pairs. I cannot see which is which, as the caps cover the silkscreen

Click to expand...

To confidently locate things on the board, you need to use the "layout" diagrams in concert with the schematics. In the HiFiEngine download, there are diagrams called "Amplifier Board View", etc. These blow up on screen quite well, and will allow you to confidently identify components. The layouts for the Advent are quite good, as they show both the copper pattern on the underside of the board as well as the top silkscreened labels. It is more common that these layout diagrams only include the underside image.

At this point, verify that you measured on the leads of C400 and C401 (yes, 100uF axial caps).

1) Emitter of Q403 at "B" should be about +0.7V. measure and tell me the reading.
2) The base of Q401 at "A" should be about -15.7V, measure and tell me the reading.

My first suspicion is something wrong with Q402. Look at this area on the pcb for cold solder, touch up with soldering iron to make sure all the soldering is good. If it's still have problem, something is wrong. Report back and we do more.

Click to expand...

This is correct EXCEPT you have base and emitter reversed on q403. The emitter is grounded, and the base should be at -.7.
Failure of the FET Q402 is a common problem and probably is here.

This is correct EXCEPT you have base and emitter reversed on q403. The emitter is grounded, and the base should be at -.7.
Failure of the FET Q402 is a common problem and probably is here.

Click to expand...

You are absolutely correct. I had my draft correct for analysis, I made the mistake when I redrawn it to make it nicer looking. It was late last night, I just want to put it up so OP can start troubleshooting. Ha ha, the wrong drawing seems to work also!!! !!!! When I wrote the post, I did not catch it.

Here is the correct drawing:

The +ve 15V is from a 78M15 voltage regulator. Q401, 402 and 403 form an inverter amp to create -15V. The voltage is set by R401 and 402. Q402 has the gate and source connected together so it becomes a constant current source ( actually sink) of about 6mA ( look at datasheet, current sink when Vgs=0 is about 6mA).

You don't have -15V, things you have to check is:

1) Base of Q403 at "B" should be about -0.7V. measure and tell me the reading.
2) The base of Q401 at "A" should be about -15.7V, measure and tell me the reading.

My first suspicion is something wrong with Q402. Look at this area on the pcb for cold solder, touch up with soldering iron to make sure all the soldering is good. If it's still have problem, something is wrong. Report back and we do more.

Attached is the image Alan requested, which is likely not needed anymore. Also attached are images from both sides of the Q402/Q403 locations. Q402 is yellow on top. Q403 is the same shape but black on top. The images are, obviously, reversed left to right.

In the pic of the back of the board, I believe the two lone solder dots at the bottom are for the small cylindrical cap. The bottom dot has no voltage (actually, about .002VDC). The top dot, and the kidney-shaped blob above it, have 25 VDC. The next biggest blob, to the upper left of the kidney-shaped blob, has about .7VDC. The smaller dot below that (which is to the right of the kidney-shaped blob) has about 7V and the dot below that (also to the right of the kidney-shaped blob) has basically no voltage. Those last few readings are going to correspond with Q403 and 402. I cannot see all the silkscreen printing, but I think Q401 is the black thing with the metal tab atop located directly above the cap. It looks different from Q402 and 403.

I wish I could offer more information but, as I say, I am new at this. And I am very appreciative of the information you folks have offered so far.